Process for the production of



United States Patent PROCESS FOR THE PRODUCTION OF REFRACTORY METALS Dale M. Hiller, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed May 29, 1958, Ser. No. 738,636 9 Claims. (Cl. 75-845) This invention concerns the removal of metal deposits, such as niobium or tantalum deposits, from the internal surfaces of a reactor without encountering contamination of the metal or damage to the reactor.

The preparation of refractory metals is frequently accomplished by the reduction of the halides of these metals. This reduction results in metal adhering to surfaces within the reaction vessel, and if sustained operation is contemplated, this adherent metal must be loosened and removed. For example, a convenient means for preparing such highly refractory metals as tantalum and niobium is to reduce the metal halide with hydrogen within an externally heated reactor, thus depositing the metal on surfaces therein. The surfaces which are provided for deposition of the metal may be a bed of small particles of the refractory metal being prepared, or they may consist of rods, plates, spheres, crystals, etc. of dissimilar materials, such as stainless steel or fused silica, which become coated with the refractory metal. In this latter case, where the deposition sites are of dissimilar materials, the metal product must be removed from the material upon which it has been deposited. Moreover, the reduction reaction always deposits some refractory metal on the walls of the reactor, and this metal must also be removed.

It is, therefore, an object of this invention to provide means for easily removing deposited metal, for example, niobium and/or tantalum deposits, from dissimilar surfaces. Another object is to remove such metallic deposits in a form that may be easily reconverted to the elemental metal.

These and other objects of this invention are accomplished by contacting the metal deposits with hydrogen at temperatures sufliciently high to form the hydride of the metal. As a result of this process, the strong, ductile metal is converted to the brittle and friable hydride. Moreover, the conversion to the hydride results in an expansion of the deposits which loosens them from the dissimilar surface so that they either fall away or are so loosely held that they can be easily removed by brushing or scraping. The removed metal hydride is unstable at high temperatures, for example, 800 C.-900 C. Therefore, it can be reconverted to the metal simply by heating at these temperatures under conditions which remove the liberated hydrogen from the immediate environment of the reconverted metal. A reaction vessel connected to an exhaust pump or to an apparatus for purging with an inert gas is suitable for this purpose.

The process of this invention can be easily performed without keeping a close control over temperature. For example, the metal deposits may be heated to temperatures at which the hydride is unstable and then cooled to below the decomposition temperature of the hydride, and preferably to room temperature in the presence of hydrogen. Under such cooling conditions the hydride is formed and the deposits are loosened from the surface to which they had previously adhered.

In a preferred embodiment, the metal deposits are 2,943,931 Patented July 5, 1960 heated to a temperature of about 800 C. to 1000 C. in an atmosphere of hydrogen and then allowed to cool to room temperature while still exposed to the hydrogen atmosphere. Under such conditions, virtually 100% of the metal will be obtained as the hydride. Alternatively, the metal deposits may be heated to about 800 C. to 1000" C. in an inert atmosphere or in vacuum, whereupon hydrogen is introduced at this temperature, and the metal is allowed to cool to room temperature while still being exposed to hydrogen, thus forming the hydride.

For a clearer understanding of the invention, the following specific examples are given. These examples are intended to be merely illustrative of the invention and not in limitation thereof.

Example I A silica tube having an internal diameter of 22 mm. was used in this example. It was provided with a porous filter which was suitable for supporting a bed of metal particles. A charge of grams of niobium metal of mesh particle size was fluidized in the reactor by the upward flow of 1.8 l./minute of hydrogen, and 30 ml./min. of argon. The temperature of the bed was held at 850 C., by external heating means, while 35.5 g. of NbCl was introduced into the reactor over a period of 3% hours. At the end of this time, the flow of argon was increased to maintain the metal bed in a fluidized condition while the flow of hydrogen was cut off, and the bed cooled to room temperature in the inert gas atmosphere. On examination it was found that niobium had been deposited on both the bed and the reactor wall above the expanded fluidized bed.

The apparatus, together with the charge of metal particles, was reassembled, and the bed fluidized by a flow of argon. The temperature of the reactor was raised to 800 C., and the gas flow was adjusted to the hydrogen and argon flows given above (1.8 l./min. of hydrogen and 30 ml./ min. of argon). These conditions were main tained for a period of A hour. The heat was then turned 0115, and the reactor cooled to room temperature. The flow of fluidizing gases was continued during cooling. When the reactor was disassembled, the reactor wall was found to be entirely free of metal deposit. The bed material was brittle and very easily broken up into smaller particles. Analysis showed the material to be almost completely niobium hydride.

Example II Under conditions similar to those of Example I, 75 g. of mesh tantalum metal was fluidized in a 22 mm. silica tube by the flow of 1.2 L/min. of hydrogen and 35 ml./min. of argon. The temperature of the reactor was maintained at 900 C. by external heating means, and 32.5 g. of TaCl vapor was passed to the reactor over a period of 4 /2 hours. At the conclusion of this time, the flow of hydrogen was stopped, and the heat was turned 01f. The flow of argon, however, was increased to a rate that would keep the bed in fluidized condition during cooling. When the reactor and its contents had cooled to room temperature, the flow of gases was stopped and the reactor disassembled for examination. The wall of the reactor was found to be coated with tantalum, and the bed particles had been enlarged by the deposition of tantalum metal. The apparatus was then reassembled; the flow of hydrogen and argon was reestablished; and the reactor was again heated to 900 C. These conditions were maintained for /2 hour after which time the reactor was cooled to room temperature while the flow of the mixed gases was continued. When the apparatus had cooled, it was again disassembled and the reactor wall was found to be clean. The bed material was found to be of increased size, and it was brittle.

Analysis showed the material to be almost completely tantalum hydride.

Example III A silica reactor tube 50 mm. in diameter was packed with satin-finish silica rods of 6.25 mm. diameter to provide surface for the deposition of metal product in the reactor. Under conditions of operation similar to those described in Example I, the reactor was flushed with hydrogen and the temperature of the reactor was brought to 850 C. The hydrogen flow was adjusted to 1.8 L/min. and over a period of 3% hours, 35 g. of NbCl was vaporized and passed into the reactor. When all of the NbCl had been vaporized and fed into the reactor, the system was slowly cooled while continuing the flow of hydrogen to the reactor. Upon disassembling the apparatus, the niobium hydride product of the reaction fell free from the rods, and the deposits on the reactor wall could be easily scraped off.

As will be seen from Example III, the present inven tion makes it possible to remove deposited metal product from the reactor walls and the specially provided deposit sites simply by flowing hydrogen through the reactor at elevated temperatures and cooling the reactor and contents in the presence of hydrogen. Such a removal process is especially convenient when hydrogen is being used as a reactant, since all that has to be done is to continue the flow of hydrogen after the metal-producing reaction is stopped.

In the above examples, the hydriding was carried out at substantially atmospheric pressures.

However, higher pressures can be used. It should also be pointed out that the invention is applicable to the removal of the deposits of any metal which forms a brittle, expanded hydride upon combining with hydrogen. The dissimilar surface from which the metal deposits are removed may be of any material capable of withstanding the conditions of operation herein disclosed.

It is possible to operate my invention under a broad range of conditions. However, the temperature and time required for optimum operating conditions will depend to some extent upon the identity, purity, and physical state of the deposits to be removed. It has been found that extremely pure deposits of certain refractory metals, for example niobium, may be hydrided at a temperature of about 300 C. or less, the reaction taking place almost instantaneously. In cases where an oxide film or other impurity has contaminated the metal deposit, it may be necessary to go as high as 1000 C. and hold the deposit at this temperature for several hours before cooling in hydrogen in order to obtain a hydrided metal which will be readily removable from the reactor.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

I claim:

1. In a process for the production of refractory metal selected from the group consisting of niobium and tantalum wherein a halide of said metal is reduced with hydrogen within a reactor to deposit said metal on dissimilar surfaces within said reactor, the improvement which comprises conducting the following steps after discontinuing the introduction of refractory metal halide into the reactor: maintaining the surfaces of the reactor at a temperature between 800 C. and 1000 C. and passing hydrogen through said reactor, cooling the reactor surfaces while in contact with hydrogen to below the decomposition temperature of the hydride, thus causing a brittle refractory metal hydride to form, and removing said hydride from the dissimilar surfaces.

2. The process of claim 1 in which the refractory metal is niobium.

3. The process of claim 1 in which the refractory metal is tantalum.

4. A process for removing metal selected from the group consisting of niobium and tantalum which has been deposited on dissimilar surfaces, comprising reacting said metal with hydrogen to form a brittle, expanded metal hydride, thereby loosening said hydride from the dissimilar surfaces, and removing the metal hydride from the surfaces.

5. A process for removing refractory metal deposited on dissimilar surfaces within a reactor comprising reacting with hydrogen a refractory metal selected from the group consisting of niobium and tantalum to form a brittle refractory metal hydride and removing said hydride from the dissimilar surfaces.

6. The process of claim 5 in which the refractory metal is niobium.

7. The process of claim 5 in which the refractory metal is tantalum.

8. A process for removing refractory metal selected from the group consisting of niobium and tantalum which has been deposited on dissimilar surfaces within a reactor comprising maintaining said refractory metal at a temperature sufficient to form its hydride, and then contacting said metal with hydrogen to form a brittle refractory metal hydride and removing said hydride from the dissimilar surfaces.

9. A process for removing refractory metal selected from the group consisting of niobium and tantalum which has been deposited on dissimilar surfaces within a reactor comprising subjecting the refractory metal deposit to a temperature above that at which the hydride will form, lowering the temperature with the metal deposit in contact with hydrogen to form a brittle, expanded metal hydride and removing said hydride from the dissimilar surfaces.

References Cited in the file of this patent UNITED STATES PATENTS 2,516,863 Gardner Aug. 1, 1950 2,604,395 Gonser et a1. July 22, 1952 2,784,054 Carter et a1. Mar. 5, 1957 

1. IN A PROCESS FOR THE PRODUCTION OF REFRACTORY METAL SELECTED FROM THE GROUP CONSISTING OF NIOBIUM AND TANTALUM WHEREIN A HALIDE OF SAID METAL IS REDUCED WITH HYDROGEN WITHIN A REACTOR TO DEPOSIT SAID METAL ON DISSIMILAR SURFACES WITHIN SAID REACTOR, THE IMPROVEMENT WHICH COMPRISES CONDUCTING THE FOLLOWING STEPS AFTER DISCONTINUING THE INTRODUCTION OF REFRACTORY METAL HALIDE INTO THE REACTOR, MAINTAINING THE SURFACES OF THE REACTOR AT A TEMPERATURE BETWEEN 800*C. AND 1000*C. AND PASSING HYDROGEN THROUGH SAID REACTOR, COOLING THE REACTOR SURFACES WHILE IN CONTACT WITH HYDROGEN TO BELOW THE DECOMPOSITION TEMPERATURE OF THE HYDRIDE, THUS CAUSING A BRITTLE REFRACTORY METAL HYDRIDE TO FORM, AND REMOVING SAID HYDRIDE FROM THE DISSIMILAR SURFACES. 